The invention relates to a method for the transmission of data with transmission error checking. More in particular, the invention is related to a method for the transmission of data between a transmitting end and a receiving end of a transmission channel, involving the generation of supplementary data at the transmitting and the receiving end by a first and a second function respectively, and the comparison of the supplementary data generated by said functions in order to detect transmission errors. The invention further relates to devices for the application of such a method.
A method and devices of the above-mentioned kind are known in the prior art. In this connection, the transmission channel can be set up for transmission in space (bridging a distance) as well as for transmission in time (storage and display). Further, the second function can be implemented in such a manner that it is, for example, an inverse of the first function, so that the original data are reconstructed. This allows the original data, rather than the supplementary data, to be compared. Reference is made to European Patent Application 0,494,036 and U.S. Pat. No. 4,961,192.
Transmission errors may arise, for example, through electromagnetic radiation, inadequacies in a storage medium (transmission in time), and errors in switching and transmission equipment. Prior art methods provide for the checking of transmitted (user) data by the addition of supplementary data which, as redundant information, enable the detection of transmission errors to a certain degree. As an example of such supplementary data, so-called parity bits, which at the transmitting end can be added to binary data by a first function and, for example, represent the number of ones and zeroes, can be mentioned. At the receiving end it can be checked whether this number of ones and zeros, as repesented by the parity bits, corresponds with the data received. If the parity bits do not correspond with the data received, a transmission error has apparently occurred and the data concerned could be transmitted again if necessary.
In prior art methods the problem arises that systematic errors are sometimes not detected. Systematic errors, that is to say, errors which repeat themselves, can arise inter alia through an error which repeats itself in the transmission channel (for example an interference signal with a certain frequency) or through an equipment error. For (binary) data it holds that there is always a probability that erroneous data are considered to be correct data because the supplementary data may be correct by coincidence (the supplementary data are always restricted in length and therefore a finite number of supplementary data can be distinguished). With systematic errors the case may therefore arise that an error, once not recognised as such, is continually not detected.
International Patent Application WO87/03442 discloses a data encryption device comprising a CRC (Cyclic Redundancy Code) generator. A counter produces count bits related to the data. The CRC generator computes a CRC on the basis of the encrypted data to which the count bits are appended. Although the appending of count bits provides an improved protection against systematic errors, such errors can still not be excluded as the data proper are not altered.
The non-detection of a transmission error in the data can make itself felt in the rest of the data received. This is especially the case if, on the transmission path, the data is compressed or otherwise encoded. In the decoding process, not only the erroneous data, but in some cases all following data may become unusable, in particular if the statistics by means of which the compression and decompression take place are affected by the erroneous data.
The ITU-standard V.42bis describes, for example, an adaptive compression scheme which, during the compression, records statistics of the data to be compressed. If systematic errors are not detected, the said statistics no longer correspond with the data, so that at the receiving end an incorrect decompression takes place. This may result in all decompressed data becoming unusable.